1. Field of the Invention
The present invention relates to a webbing driving apparatus for use in a seatbelt system for a vehicle which is so designed that the distal end protion of an occupant restraining webbing is guided by a guide rail mounted on the body of the vehicle, thereby allowing the webbing to be automatically fastened to and unfastened from the occupant's body.
2. Description of the Related Art
There is one type of seatbelt system, known as "automatic seatbelt system", in which, when an occupant restraining webbing is driven, it is guided by a guide rail mounted on the vehicle body so as to be automatically fastened to and unfastened from the occupant's body.
One type of apparatus for driving the webbing in such an automatic seatbelt system is arranged such that one end of a line means such as a wire is connected to the webbing, while the other end of the line means is connected to a drum, and the webbing is driven by winding up the line means onto the drum or unwinding the former from the latter, as shown in the specification of Japanese Patent Laid-Open Publication No. 7926/1974 (German Application No. P2216709.5).
This type of webbing driving apparatus suffers, however, from the disadvantage that, when the line means is not regularly wound up onto the drum, it may be impossible to wind up a predetermined amount or length of the line means. In order to cope with this problem, a structure for webbing driving apparatus such as that shown in FIGS. 5 and 6 may be adopted. Namely, the outer peripheral surface of an inner drum 10 is tapered, and a tapered groove is formed inside an outer drum 12, thereby defining a guide groove 16 between the inner and outer drums 10 and 12 for guiding the line means 14 toward the drum shaft.
In this structure, however, when no tension acts on the line means 14 when unwound, the portion of the line means 14 which is wound on the inner drum 10 may be loosened and thereby disordered. In such a case, variations are produced in the density of turns of the line means 14 wound on the inner drum 10. In a low density portion, a gap is produced between each pair of adjacent turns of the line means 14, so that, when the line means 14 is driven, the adjacent turns of the line means 14 may collide with each other across this gap and generate a noise.